With the spread of wireless devices such as mobile phones, Doherty amplifiers have been increasingly adopted as one design of high-efficiency technology for high-frequency amplifiers. A Doherty amplifier comprises two amplifiers, namely, a carrier amplifier and a peak amplifier. A Doherty amplifier can adjust output levels by causing only a carrier amplifier to operate in regions of low output levels, and causing the carrier amplifier and a peak amplifier to operate when an output level approaches the saturation region of the carrier amplifier, thereby widely providing highly efficient output levels.
However, retaining highly efficient output levels throughout a broad frequency band is difficult with Doherty amplifiers, therefore, broadening the bandwidth of Doherty amplifiers has become an issue. The conventional techniques for broadening the bandwidth include, for example, configuring the output line of a peak amplifier to have the same characteristic impedance as the output line of a carrier amplifier and in the length corresponding to the half-wavelength of an operating frequency. There is also a technique of forming the output line of a peak amplifier by two types of lines.
However, these conventional techniques allow the impedance from a combining point on the peak amplifier side to grow over a broad frequency range, and the efficiency in a broadband cannot improve.